1. Field of the Invention
The present invention relates to a semiconductor device, and a method for manufacturing such a device. More particularly, the invention relates to a semiconductor device, such as a high-performance thin-film crystalline solar cell using a thin-film crystal which can be formed on a low-cost substrate, and to a method for manufacturing such a device.
2. Description of the Related Art
Solar cells (solar cell devices) are widely being studied as energy sources for driving various kinds of apparatuses or as electric power supplies subjected to system interconnection with commercial electric power. It is desirable for a solar cell device to be formed on a low-cost substrate in order to reduce production cost. Silicon is generally used as a semiconductor constituting a solar cell. Particularly, single-crystal silicon is superior from the viewpoint of efficiency in converting optical energy into electric power, i.e., the photoelectric conversion efficiency. On the other hand, amorphous silicon is advantageous from the viewpoint of providing a large area and a low cost. Recently, polycrystalline silicon has come to be used in order to obtain a cost as low as amorphous silicon and an energy conversion efficiency as high as single-crystal silicon.
In methods for manufacturing semiconductor devices using single-crystal or polycrystalline silicon, it is difficult to reduce the device's thickness to a value less than 0.3 mm because a plate-shaped substrate is provided by slicing a bulk crystal. A substrate obtained by slicing a bulk crystal has generally a thickness greater than a thickness necessary for absorbing incident light (20 .mu.m-50 .mu.m), so the material is not efficiently utilized in the direction of the thickness. Recently, a method for forming a silicon sheet according to a spinning method in which a fused silicon liquid is poured into a mold has been proposed. Nevertheless, the minimum thickness obtained in this method is about 0.1 mm-0.2 mm. Accordingly, there is still room for further decreasing the thickness of silicon to reduce the amount of silicon and realize a low cost. If the thickness of crystalline silicon is reduced, it is possible to reduce the weight of a semiconductor device, such as a solar cell or the like, to provide a flexible device if necessary, and to widen the range of application. Particularly when using silicon for a solar cell, a thin crystal is more resistant to shock from the outside, such as hail or the like, than a thick crystal, and is therefore preferable from the viewpoint of increasing reliability in practical application.
As described above, a technique for providing a high-quality thin crystal, particularly a single crystal, which allows the manufacture of a semiconductor device, such as a solar cell or the like, having a high performance with a low cost is desired.